1. Field of the Invention
The invention is based on an electromechanical locking device for a brake piston of a hydraulically actuated wheel brake.
2. Description of the Prior Art
One such electromechanical locking device is know for instance from German Patent Disclosure DE 10 2005 055 084 A1. The wheel brake disclosed in this reference is hydraulically actuatable. The wheel brake has a hollow-cylindrical brake piston, whose interior is filled with hydraulic pressure fluid. In the event of brake actuation, a hydraulic pressure is built up by the driver of a vehicle. A pressure force causes a displacement of the brake piston relative to a brake cylinder, in which this brake piston is movably guided. The brake piston presses a brake lining against a rotating brake body, such as a brake disk, and the magnitude of a frictional force that comes to be established between the rotating brake body and the brake lining is proportional to the brake pressure. The frictional force brakes the brake body.
In order to brake a stopped vehicle firmly on the spot in a parking situation, the known wheel brake additionally has an electromechanically actuatable locking device. This locking device includes a spindle that can be driven to execute a rotary motion by an electric drive motor and an intervening gear. The spindle in some portions protrudes into the interior of the brake piston and has a spindle nut. This spindle nut is disposed in a manner fixed against relative rotation, and as a result, a rotary motion of the spindle trips a translational motion of the spindle nut. In one of its terminal positions, the spindle nut is in contact with the brake piston and acts upon it with an axial force. With this axial force, the brake piston acts on the brake lining and presses it against the stationary brake body and thus prevents the latter from being able to execute a rotary motion.
The spindle and the spindle nut occupy a considerable portion of the internal volume of the hollow-cylindrical brake piston. Nevertheless, the remaining space inside the brake piston, which is filled with hydraulic pressure fluid, is relatively large. This has the disadvantages that a comparatively large amount of pressure fluid has to be positively displaced by the driver in order to build up brake pressure, and the response performance of such a wheel brake is therefore not fully satisfactory. In professional circles, such a response performance of a wheel brake is called indirect.
Another disadvantage is that hydraulic pressure fluid does not behave ideally incompressibly, and that therefore with increasing pressure fluid volume in the brake piston, the hydraulic elasticity of a brake circuit also increases. This elasticity is expressed as an unwanted, soft actuation of the service brake.
Moreover, the spindle nut received in a manner fixed against relative rotation is urged axially by the rotationally driven spindle, in its terminal positions, in such a way that mechanical warping can occur between the individual components. This warping makes an ensuing release of the spindle nut from its terminal positions more difficult or even impossible, so that an actuated locking device might still remain blocked, and then the vehicle cannot be moved from that spot.